1. Field of the Invention
The present invention relates to dielectric motors; and in particular to dielectric motors that use charged moving components, or a rolled electro-active polymer (EAP) to generate rotating mechanical motion, or both, and vehicles that employ such motors.
2. Description of the Related Art
Conventional electric motors and combustion engines are widely used in commerce but are not appropriate for all applications. For example, there is use for a class of balloon vehicles that supports a payload at high altitude, e.g., above 100,000 feet, at a relatively fixed geographic location for extended periods of time. The high altitudes may include the stratosphere layer of the atmosphere or the atmosphere of a different planet, such as Mars. Such a vehicle can support a communication terminal or surveillance system that covers a much wider area than can be served by a tower or a winged aircraft and gives more temporal coverage and detail than can be achieved from an orbiting satellite. To maintain geographic position in the presence of high altitude winds, where the air has low density, a motor is useful that can turn a station-keeping propeller that is both large and slowly rotating. Another class of vehicles includes spacecraft and satellites.
A desirable motor would be capable of providing one to several horsepower. For extended duration, the motor should be highly efficient, turning most of the energy available on board into mechanical rotation and dissipating very little as heat. To leave capacity for payload, the motor should be light, e.g., less than fifty pounds including power supply. In applications that benefit from security against attack from hostile entities, the whole platform, including the motor, should have small radar reflectivity (i.e., low radar cross-section), e.g., be as transparent to radar as plastic materials.
Combustion engines explode fuel in chambers in which a directional element, such as a piston or rotor, moves. Such engines dissipate a great deal of energy as heat, require massive engine blocks to withstand the explosions, are made of metals that have high reflectivity to radar, and require oxygen that may be in short supply at high altitudes. The oxygen requirement, alone, makes combustion engines unsuitable for some vehicles, such as satellites and other spacecraft.
Electric motors include alternating current (AC) motors, direct current (DC) motors, servo motors, and stepper motors. Electric motors utilize magnets which are made of metal, and are therefore relatively massive and have high radar reflectivity. AC motors are limited to a few speeds that are related to the AC frequency of an AC power supply. The speeds are relatively high, over a thousand revolutions per minute (rpm). To attain different speeds, a gearbox is attached to the driveshaft. These motors draw substantial current and therefore require relative heavy sources of electrical power, such as AC generators and large banks of batteries.
Compressed gas and hydraulic motors require a source of gas or hydraulic fluid and have poor energy efficiency as chemical energy is converted first to pressure and then to mechanical movement.
A new class of motors uses electroactive polymers (EAPs) to convert from electrical to mechanical energy. When a voltage is applied to electrodes in contact with an EAP, the EAP deforms. An EAP sandwiched between stretchable electrodes deforms in two dimensions as described in U.S. Pat. No. 6,664,718 by Pelrine et al., entitled “Monolithic Electroactive Polymers” (hereinafter, Pelrine I). In EAP motors, the deformation is leveraged to provide mechanical motion. Metallic content can be reduced substantially in such motors. The electrodes can be thin or formed with non-metallic conductors, such a carbon nanotubes. The transfer from electrical energy to mechanical energy is quite efficient.
Linear EAP motors are described, for example, in U.S. patent application US 2002/0008445 by Pelrine et al., entitled “Energy Efficient Electroactive Polymers and Electroactive Polymer Devices” (hereinafter, Pelrine II). Such motors are quite useful in some applications that rely on linear motion, as in robotic appendages. However, such linear motors suffer some disadvantages. Linear motors often require motion-constraining components to direct two-dimensional deformation from a sheet of EAP material into one dimension. Such components add to complexity and are subject to wear and breakdown. Such linear motors also fail to take advantage of a wide range of well established technologies to use rotating drive shafts, such as are common in the automobile and aircraft industries.
Rotary motion provided by flat sheets of EAP sandwiched between stretchable electrodes are described in U.S. patent applications US 2002/0175598 by Heim et al., entitled “Electroactive Polymer Rotary Clutch Motors” (hereinafter, Heim I) and in U.S. patent application US 2002/0185937 by Heim et al., entitled “Electroactive Polymer Rotary Motors” (hereinafter, Heim II). Such motors can take advantage of the well established technologies that use rotating drive shafts.
However, the motors described in Heim I and Heim II provide low power—well less than one horsepower, and therefore insufficient to move a propeller such as envisioned for some applications. Furthermore, the described motors involve numerous joints and extensions to convert small stroke linear deformations into rotating movement; such joints and extensions add to complexity and are subject to wear and failure. For example, Heim I requires use of a clutch to engage a range of motion in one direction (a stroke) and disengage during a return stroke. In addition, the space required for long beams to extend from the EAP to a rotating shaft increases the size and weight of the motor. Furthermore, the sheets in Heim II require relative large size to achieve a useful stroke and a relatively large volume to provide a unit force. Joints and pins located at one point in the sheet are subject to large stresses and can be expected to be a point of failure, such as a source of a fissure in the sheet.
Based on the foregoing, there is a clear need for a lightweight, low-metallic motor with sufficient horsepower that does not suffer the disadvantages of prior art motors.